Translation of PCT/EP01/09364 as filed on Aug. 14, 2001.
The invention concerns a displacement detecting element comprising a measuring resistance, a voltage source, and a displacement-dependent voltage pick-off, wherein the measuring resistance is in the form of a strip and this resistance strip is at least as long as the maximum displacement length of the structural part to be detected.
The invention also concerns a multi-leaf collimator with displacement detecting elements of this type.
The principle of function of displacement detecting elements of this type is known (DE 38 38 662 A1). Those displacement detecting elements are potentiometers which are too large to be able to be directly disposed on the leaves of a multi-leaf collimator for detecting its displacement. Multi-leaf collimators are used to define a high-energy beam for tissue irradiation and have a plurality of thin, laminar leaves, which delimit and shape the beam from two sides. The leaves must be individually and exactly positioned, which requires precise detection of their positions. Conventional potentiometers do not permit detection of the position directly on the leaves due to the limited space and the disturbing influence of the radiation.
The company leaflet xe2x80x9cMotorized Micro Multileaf Collimator 06/98xe2x80x9d of the company Leibinger discloses arranging drives alternately above and below and opposite to the leaves in an offset and fanned-out fashion due to the limited space at the densely packed thin leaves. Each of these drives has an associated potentiometer, which serves as a displacement-detecting element. In this fashion, the displacement of the leaves is indirectly detected via determination of the position of the leaf drives.
In addition to the space requirements of the potentiometer, which increases the size of the collimators, this indirect measurement has the disadvantage that the error tolerances of the drive elements also falsify the position detection and the hysteresis produced by the play of the drive elements must be considered when the position detection adjusting direction is reversed. Since the adjusting motion must be extremely precise to protect the patient, the error tolerances of the drive elements must be kept extremely small and the hysteresis must be compensated for. In addition to its size, the stationary hysteresis-compensated arrangement of the displacement detecting elements in the above-mentioned prior art has the disadvantage that the possibilities for disposition on the leaves are limited. Moreover, if such an electrical displacement-detecting element is subjected to increased radiation of electrically charged parts, measuring errors may be produced by radiation-induced currents.
It is therefore the underlying purpose of the invention to design a displacement-detecting element of the above-mentioned type such that the position of the leaves of a multi-leaf collimator can be precisely detected, without errors.
This object is achieved in accordance with the invention in that the displacement detecting element detects the displacement of the leaves of a multi-leaf collimator by securely connecting either the measuring resistance or voltage pick-off to the leaf to be detected, with the other functional element being fixed, and at least these two functional elements are disposed in a region of the leaves which is not exposed to the main radiation.
This object is achieved with the multi-leaf collimator in that each leaf is provided with at least one displacement-detecting element of this type and all displacement detecting elements can be connected to the multi-leaf collimator control.
The invention permits position detection directly at the very thin leaves although these are densely packed next to each other without sideward gaps. In accordance with the invention, this arrangement is moreover provided in a region of the leaves, which is subjected to only relatively low radiation since the main radiation is shielded in this region. There are two possible principles of shielding which will be explained in further developments of the invention. Direct mounting of the measuring resistance or the voltage pick-off on the leaf prevents propagation of measurement errors by transmission elements and hysteresis must not be taken into consideration when the motion is reversed. A secure arrangement on the structural part to be detected is effected through direct mounting of a functional element (measuring resistance or voltage pick-off) or secure mounting of a carrier or housing. This produces an extremely high precision in a simple and inexpensive fashion.
When the multi-leaf collimators are provided with inventive displacement detecting elements, a high measuring accuracy in the region of {fraction (1/10)} to {fraction (2/10)} mm is easily obtained. This provides high precision and safety for intensity-modulated radiation therapy. The collimators can also be decreased in size since there are no potentiometers disposed on the drives.
The displacement detecting elements can be securely connected to the leaves without hysteresis via an extension on their rear side. It is particularly advantageous to directly connect them to the leaves, since the space on the rear side of the leaves thereby remains free.
Arrangement of the device components in a region of the leaves which is not exposed to the main radiation is effected by disposing the functional electrical elements of the displacement detecting element on that side of the leaves facing away from the beam to thereby shield the main radiation with full leaf thickness.
Since the leaves must have a certain length to be guided safely, they are only partially loaded with the main radiation. This permits arrangement of the electrical functional elements in the rear region of the leaves, which is not subjected to the main radiation. In contrast to the prior art arrangement, the thickness of the leaves is advantageously thereby not increased by the arrangement of the displacement detecting elements. The shielding effect of the leaves can be weakened in this region via an appropriate recess, since the leaves need not shield the main radiation at this location. This rear region of the leaves is usually shielded from the main radiation by a pre-collimator, which consists of two pairs of shielding blocks, forming a rectangle. This shielding may, of course, also be provided by a window at the radiation source.
A further development of the invention provides simple and precise adjustment of the displacement-detecting element through compensation by partial removal of material of the measuring resistance strip.
The displacement-detecting element may be directly mounted to the leaf. Since the leaves are made from radiation-absorbing metal, a non-conducting base layer must be initially disposed onto which the resistance layer is arranged. It is also possible to provide the displacement detecting element with a separate, preferably thin housing, which can be directly and rigidly mounted to the leaf. The housing may e.g. be inserted into a recess of the leaf. The housing may be designed such that it defines a recess extending along its length. A fixed tongue is displaceably disposed in this recess, wherein the displacement corresponds to at least the length of displacement of the leaf. The housing and tongue are thereby each associated with one of the functional elements, i.e. the measuring resistance or the voltage pick-off. The strip of the measuring resistance may be disposed e.g. on the tongue and the voltage pick-off on the housing. Ideally, the housing and tongue are designed as flat plastic parts. The electrical functional elements, which have at least the maximum length of displacement performed by the leaf, are located in a gap between the housing and tongue and are therefore well protected and wear and inaccuracy due to mechanical loading is prevented. In addition to the measuring resistance, the functional elements may include a conductor path for connection of the measuring resistance and a second slide contact path.
In a possible embodiment, the measuring resistance is connected to the voltage source by disposing a terminal at one end of the resistance strip and a conductor path communicates with the other end and extends parallel to the resistance strip to the other terminal of the voltage source. The resistance strip and the conductor path may moreover be disposed on the tongue wherein the resistance strip is formed as first slide contact path and a second slide contact path with a terminal is disposed parallel thereto, wherein voltage is picked-off by disposing two electrically connected wipers on the housing which slide on the slide contact paths and with means disposed between the terminals of the second slide contact path and the voltage source for converting an associated signal into displacement information.
The above-mentioned arrangement provides for a particularly advantageous further development when the wipers are disposed on the outer side of the housing and penetrate through a window in the housing to slide on the slide contact paths. This design provides a particularly flat housing potentiometer. The height can be 1.4 mm or even less. This extremely thin construction is useful in particular for the above-mentioned use in multi-leaf collimators since increasingly thinner leaves are being used to obtain more precise formation of the shape of the region to be irradiated.
Measurement may be performed e.g. in that the means for converting the signal into displacement information is a voltage meter which is calibrated to the displacement to be detected and which picks off the measured voltage at a resistance disposed between the terminal of the second slide contact path and one of the terminals of the first slide contact path or the conductor path.
If the displacement detecting element is provided with its own housing as described above, a shielding conducting layer may be disposed on its outer surface which may be set to a certain potential, e.g. the potential of the structural component, or be grounded.
In order to create very flat displacement detecting elements and/or avoid space needed for screws, the mounting is advantageously effected by providing the housing with a dovetailed outer contour, which is inserted into a complementary recess of the leaves.
The displacement-detecting element may of course be spatially designed in the most different ways in dependence on the intended use. The actual displacement detecting element circuit may also have different designs. It is essential to the invention that the functional elements are directly mounted on the leaf to be measured and are designed such that they require little space, i.e. are very flat. Direct mounting to the leaves of a multi-leaf collimator is advisable to optimize position measurement. This also permits rapid and exact change of the contours of the regions to be irradiated as defined by the multi-leaf collimators. In this fashion, a region, e.g. a tumor, can be irradiated from different directions with a rapidly and precisely adjusted shape and without time delays caused by correction of hysteresis errors when the motion is reversed.
The invention is explained below with reference to an embodiment shown in the drawing.